Ambient environmental air in a home, office, educational, institutional, industrial or institutional setting can be a contributing factor in maintaining a healthy environment. Particulates, such as pollen, dust, mold, spores, bacteria, viruses, animal dander, skin cells, or the like, and volatile chemicals, including volatile organic compounds, commonly referred to as VOCs, formaldehyde, cleansers, pesticides, fungicides, combustion by-products, odors and toxic gases are frequently present in the ambient air. These airborne elements have been implicated in a wide variety of respiratory conditions and diseases.
Often times, the indoor level of air pollution can be exponentially higher than outdoor levels because of the sealed nature of the indoor environment. Given that the majority of people spend most of their time indoors at school, work or at home, control over the ambient air quality may be a significant factor in maintaining good health.
A wide variety of air treatment devices and methods are known in the art. One approach employs HEPA (High Efficiency Particulate Air) filters which can collect almost 100% of airborne particles greater than 0.3 microns in diameter. Another approach involves electrostatic precipitators which use electrostatic forces to remove particles from the air and is able to collect particles down to a diameter of 0.01 microns. However, neither HEPA filters nor electrostatic precipitators have demonstrated ability to remove volatile organic compounds (VOCs) from the air, and thus can do nothing to reduce odors or the health effects of the VOCs in the air.
One approach for treating air involves photocatalytic oxidation (PCO) technology. PCO technology has been used to remove organic contaminants and compounds from air fluid streams. In commonly used institutional air filtration systems that incorporate PCO technology, the PCO system used generally include one or more ultraviolet (UV) energy sources for irradiating UV light onto a substrate with a titanium dioxide coating. Disintegration of organic compounds takes place through reactions with oxygen (O2) and hydroxyl radicals (OH). The O2 and OH reactions with VOCs drive these diverse gas-phase odor causing contaminants to change their chemical make-up, thereby reducing odors.
While complete oxidation without a photocatalyst is thermodynamically possible, this process is kinetically slow and is not, therefore, institutionally feasible. Although various photocatalysts have been used in PCO air filtration systems, titanium dioxide (TiO2) remains the most popular and most prevalent because of advantageous characteristics. Titanium dioxide is generally accepted as a light, strong, and anti-corrosive compound that, if scratched or damaged, will immediately restore the oxide in the presence of air or water.
Currently, there is still not a complete picture of the exact mechanism by which titanium dioxide, oxygen (O2) and UV light oxidize organic compounds. Nonetheless, experimental evidence from reaction rates at varying concentrations suggests that the oxidation reaction takes place on the photocatalyst surface. When excited by radiation, titanium dioxide photocatalysts generate electron/hole pairs which act as strong oxidizing agents to adsorbed species. In particular, light with a wavelength less than 400 nm (radiation in the near-UV range) forms these electron/hole pairs in titanium dioxide which are capable of initiating the oxidation reaction with adsorbed molecules. There is evidence from several studies that the reaction proceeds through several intermediates (depending on the complexity of the molecule being oxidized), rather than instantaneously being oxidized to carbon dioxide (CO2). It is this procession through intermediates that makes identification of the specific reaction difficult.
A titanium dioxide film layer is known to have a high refraction ratio. When a titanium dioxide film layer is irradiated by UV light of less than 400 nm, the band gap energy (the level of energy photons needed to be able to free electrons from their atomic bonds) is exceeded. When this occurs, electron/hole pairs and hydroxyl radicals (OH) are created, which causes airborne contaminants, i.e. volatile organic compounds (VOCs), particulates, and bioaerosols, to be attracted to the titanium dioxide. These contaminants are eventually oxidized through a procession of reactions and intermediates concluding with CO2 as the primary end product.
Although the use of titanium dioxide as a photocatalyst for PCO air filtration systems has been widespread, other photocatalysts have also used been used. These photocatalysts include stannic oxide, zinc oxide, vanadium oxide, dibismuth trioxide, tungsten trioxide, ferric oxide, strontium titanate, cadmium sulphide, zirconium oxide, antimony oxide, and cerium oxide. The use of these photocatalysts for powder coating onto the surface of a substrate is within the scope of the invention
Because of the major role played by the photocatalyst in an oxidation reaction, many improvements to the efficiency of the air filtration process can be made through the photocatalyst itself. One determinant of PCO efficiency involves the form of the photocatalyst used. The photocatalyst titanium dioxide (sometimes also referred to as titania and titanium oxide) can exist in four forms: rutile, anatase, brookite, and as titanium dioxide (B). In the rutile form, titanium dioxide exists as a tetragonal mineral usually of prismatic habit. In the anatase form titanium dioxide exists as a tetragonal mineral of dipyramidal habit. In the brookite form, titanium dioxide exists as an orthorhombic mineral. Lastly, titanium dioxide can also exist in the form commonly referred to as titanium dioxide (B), which is formed of monoclinic materials.
Studies have been done using the high-surface area Degussa P25 form of titanium dioxide (surface area of 50 m2/g) to be highly effective in oxidizing airborne contaminants. Since the oxidation reaction rate is proportional to the photocatalyst surface area, increasing the effective surface of the titanium dioxide film layer can be advantageous.
Another way of improving the efficiency of the oxidation reaction involves improving the activity of the photocatalyst itself. Studies have been conducted on the effects of loading the photocatalyst surface with different metals. In one study, silver nitrate and sodium carbonate were added to a titanium dioxide slurry and dried baked, adding to the titanium dioxide surface. This addition process can greatly increase the efficiency of the PCO process. (Kondo and Jardim, 1991).
Other studies have made similar findings. In one study, it was found that adding platinum, palladium, or gold to the titanium dioxide surface increases oxidation rates by a factor of 3 to 5 and similar results have been obtained by doping the titanium dioxide with tungsten oxide. (Wilkins and Balke, 1994).
The lifetime of the photocatalyst is another important consideration with respect to the economic feasibility of the PCO process. Even though titanium dioxide is a fairly inexpensive compound, frequent replacement would be inconvenient and would decrease the economic viability of a PCO air filtration system. In general, photocatalyst activity should remain relatively constant with moderate use of a PCO air filtration system. However, a major barrier to a long lifetime of the photocatalyst is the adsorption of non-organics, taking up reaction sites on the photocatalyst surface.
Another major barrier to the lifetime of the photocatalyst involves susceptibility of the photocatalyst to chipping and flaking. Often a layer of titanium dioxide coated onto a substrate using a conventional liquid coating process will chip and flake off with repeated use of the PCO air filtration system. Replacing the titanium dioxide coated substrate can be burdensome.
Another relevant component to the PCO air filtration process involves the illumination source. It has been found, in particular, that 253.7 nm ultraviolet (UV) light applied from 36 watt UV lamps exhibits particularly good photocatalytic oxidative activity when used with the present invention. Similar positive results have also been found with 60 watt and 8 watt UV lamps that emit UV light at approximately 253.7 nm. Depending on the particular air filtration applications involved, other UV lamps emitting light at other UV wavelengths and at other power levels are also within the scope of the invention.
Exemplary known PCO air purification systems are represented by U.S. Pat. Nos. 6,884,399, 6,833,122, and 6,716,406 (hereinafter collectively referred to as “Reisfeld”), which describe a PCO air purifier using an aluminum filter substrate coated with titanium dioxide particles. Reisfeld also describes a control system that includes a motion detector and a sensor for detecting contaminant concentration. Nevertheless, Reisfeld does not disclose an air filtration system, wherein titanium dioxide particles are powder coated onto a substrate.
U.S. Pat. Nos. 6,838,059, 6,835,359, and 6,508,992 (hereinafter collectively referred to as “Taoda”) also describe a PCO air filtration system. More specifically, Taoda describes coating titanium dioxide (in its anatase form) to increase the specific surface area of the photocatalyst pellets. Furthermore, Taoda also describes coating titania sol by suspending fine titanium dioxide particles in water.
U.S. Pat. No. 6,797,127 to Murata et al. (hereinafter referred to as “Murata”) discloses a PCO system, wherein titanium dioxide “is directly adhered to a substrate by a low temperature flame spray coating process without using a binder, and a sol-gel process, that is, a process wherein a sol comprising a photocatalyst (photo-semiconductor particles, and an adsorbing material), a film forming component as inorganic binder, and a solvent is adhered on a substrate and then the sol is gelatinized, for example, at 300 to 400° C.” Murata's flame spray coating process involves spraying titanium dioxide onto the substrate “by a gas flame spray coating process using oxygen, acetylene, or the like, together with a ceramic melted at about 2900 to 3000° C.”
U.S. 2006/0182670 to Allen (hereinafter referred to as “Allen”) also discloses a PCO system. Allen describes the difficulties of depositing titanium dioxide on a surface because this process creates an unstable material. More specifically, Allen notes that the titanium formed from this type of process is prone to flaking off from the surface, and thus would not be able to withstand washing. Allen proposes fixing titanium dioxide on the substrate through impregnation. One of Allen's suggested methods involves impregnating the titanium dioxide while melt-spinning fibers to produce a doped fiber. Another suggested method by Allen involves coating a fiber with titanium dioxide and running it through a heated region to anneal the titanium dioxide to the fiber.
While there currently exist PCO air filtration systems, the current existing systems suffer deficiencies. Heretofore, the preferred process for coating a substrate with titanium dioxide involved conventional liquid coatings, such as a sol-gel process. However, these known processes for coating a substrate with titanium dioxide using conventional liquid coatings suffer drawbacks. For one, a conventional liquid coating will often have longitudinally running and sagging streaks on the substrate. These longitudinal streaks generally become more pronounced with increased coating thickness. The occurrence of longitudinal streaks can cause quality problems in the form of striation, i.e., longitudinal non-uniform coating thickness caused by uneven application of the liquid coating and by longitudinal streaks. It is known in the art that with conventional liquid coatings, horizontally coated surfaces and vertically coated surfaces can have different surface characteristics, e.g., different coating thickness. Thus, the process of coating a substrate with titanium dioxide using conventional liquid coating presents challenging operational constraints, particularly when thicker coatings are desired.
Furthermore, titanium layers formed on a substrate through a conventional liquid coating process have been found to have high incidence of chipping and flaking.
Accordingly, there is a need for a system and method for PCO air filtration using a substrate, wherein titanium dioxide is coated onto the substrate, without the above-mentioned operational constraints, thereby creating a layer of titanium dioxide with substantially uniform thickness. There is also a need for a PCO air filtration system that will effectively destroy bioaerosols (including microorganisms such as bacteria, viruses, mold and dust mites), volatile organic compounds (VOCs including chemical gases, solvents, and odor causing contaminants), and yet is durable and will have a long lifetime.
The discussion of art in this section is not intended to constitute an admission that any patent, publication, or other information referred to is “prior art” with respect to the invention, unless specifically designated as such.